Our overall objective is to characterize endogenous guinea pig viruses (GPV) and determine their role in leukemia. We have previously shown that endogenous viruses can be induced by bromodeoxyuridine and iododeoxyuridine treatment of cultured cells. We have further shown that proviral DNA is present in all guinea pig cells induced or not and viral induction is due to the activation of proviral DNA. We have determined the state of proviral DNA in quinea pig cells, we have shown that part of the viral DNA homologous to one-third of the viral genome is repeated 6-10 times, whereas the rest of the proviral DNA representing two-thirds of viral genome is present only 1-2 times per haploid guinea pig cell. We are now attempting to determine the expression of GPV DNA in normal and cancer tissues by using techniques of molecular hybridization.